Quaternary onium salts are highly soluble and readily dissociate in many non-aqueous as well as aqueous solvents. This property enables many solvents, which conventionally are not considered as having the ability to support electrical conductivity, to be used as an effective electrolyte medium.
Quaternary onium cations, and in particular tetraalkylammonium cations, are relatively electrochemically stable. That is, these cations are relatively stable against electrochemical reduction processes. When these cations are combined with a similarly stable anion (i.e. stable against electrochemical oxidation processes) in a suitable solvent, the resulting electrolyte can be used in a wide variety of electrochemical applications such as carbon-based electrode supercapacitors. See e.g. U.S. Pat. No. 5,086,374, which is incorporated herein by reference.
The electrochemical stability of a substance is represented by its so called electrochemical window, possessing Volts (V) as a unit and determined by cyclic voltammetry (CV). The higher the value in Volts (or “the wider” the electrochemical window), the more stable the corresponding material is against oxidation or reduction processes. The general trend of cation stability is:pyridinium<pyrazolium≦imidazolium≦sulfonium≦ammonium,and the trend of anion stability is:halides (I−, Br−, Cl−, F−)<chloroaluminates≦perfluorinated ions (e.g. BF4−, PF6−, AsF6−) ≦([CF3SO3]−, [(CF3SO2)2N]−, [(C2F5SO2)2N]−, [(CF3SO2)3C]−.The wideness of the electrochemical window is generally very sensitive to impurities, especially residual halides. Since halides are generally oxidized much easier than, for example, perfluorinated anions, halide contaminants in an electrolyte can frequently lead to significantly lower electrochemical stability.
Further “free acid” has been identified as a critical parameter, e.g. low ppm-levels of tetrafluoroboronic acid in onium tetrafluoroborate electrolytes. By applying a certain voltage to an acid containing electrolyte, the free acid equilibrates with gaseous compounds causing problems, e.g., reduced lifetimes, in the devices that use such products.
Many quaternary onium salts are commercially prepared by reacting alkylamines with alkylfluorides, alkylchlorides, alkylbromides, or alkyliodides to form alkylammonium halides. These alkylammonium halides are then converted into quaternary onium salts essentially free of fluoride, chloride, bromide, or iodide via methods known in the art, such as the one described in WO 2004/039761, which is incorporated herein by reference. However, as previously stated, chloride, fluoride, bromide, and iodide anions as well as residual acid are not electrochemically stable and even residual amounts of these substances in a quaternary onium salt reduces the overall electrochemical stability of the related electrolyte. This reduction in stability, in turn, leads to a shortened lifespan of many products that utilize such electrolytes. In addition, a residual amount of fluoride anions remaining in a quaternary onium salt would also cause corrosion, thus further reducing product lifespan.
Methods for reducing the halide and acid concentration of quaternary onium salts, even to negligible levels, are known in the art. For example, multiple crystallization steps may be employed to reduce impurity concentrations to below 5 parts per million (ppm). However, these and other known processes are time-consuming or have a low yield and are, therefore, expensive. Applicants have recognized the need for a less costly method of removing impurities such as halide and acid impurities from quaternary onium salts, preferably with a purification effectiveness that is equal to or better than the effectiveness of previously known methods.